Technology for scanning a laser beam along a linear scanning line is widely used in laser printers, facsimile machines, laser machining devices, etc. For example, there are known solar cells for solar power generation such as thin-film solar cells and flexible solar cells (hereinafter, for the sake of convenience, these solar cells are collectively referred to as “thin film type solar cells”), and in the production process of such a thin film type solar cell, a laser machining device is used to perform patterning with a laser beam on work that is a substrate having a semiconductor film such as a metal film or silicon film formed on its one surface. Examples of the substrate of the work include a rectangular glass substrate having a predetermined length and a flexible substrate used in a roll-to-roll process. In the patterning using such a laser machining device, a laser beam is scanned along a scanning line set on work, such that a thin-film layer is partially removed from the work along the scanning line, and thereby a machining line is formed. The thin-film layer that is left after the patterning is performed forms a thin film type solar cell. It should be noted that, generally speaking, a pulse laser beam is applied as the laser beam of laser machining devices for use in patterning in consideration of for example, ease of micromachining and a thermal influence reduction effect. In a case where a pulse laser beam is applied as the laser beam, the pulse laser beam is scanned such that the radiation range of the laser beam oscillated at one timing partially overlaps, on the work, with the radiation range of the laser beam oscillated one pulse width prior to the laser beam oscillated at the one timing. In this manner, the continuity of the machining line is assured. It should be noted that an area where the radiation ranges of respective laser beams of adjoining two pulses overlap with each other may be referred to as an “overlap margin”.
As described above, an optical scanning device configured to scan a laser beam is applied to laser machining devices, laser printers, and the like. Basically, such an optical scanning device is configured to: cause laser light emitted from a light source such as a laser oscillator to make angular movement by means of a deflector such as a polygon mirror or a galvano mirror; and irradiates an irradiated surface with a beam of the laser light that is making the angular movement. As a result, the laser beam is linearly scanned on the irradiated surface. Generally speaking, in order to obtain high operational reliability, the deflector is driven at a constant speed. This consequently allows the laser light to make the angular movement at a constant speed. However, if a laser beam is scanned along a linear scanning line with such a configuration, there occurs a difference in terms of laser beam scanning speed between the vicinity of the ends of the scanning line and a middle portion of the scanning line. In a laser machining device, such a scanning speed difference results in variation in size among overlap margins. Such variation in size among overlap margins causes uneven machining. A well-known optical element that eliminates the difference is an fθ lens. However, designing an fθ lens requires highly technical know-how. In addition, the size of equipment and tools for use in producing an fθ lens is limited, which makes it difficult to increase the size of the fθ lens. In view of these, conventionally, various optical elements to be used instead of an fθ lens for realizing both constant-speed deflector operation and constant-speed laser light scanning have been developed.
For example, Patent Literature 1 discloses a spherical mirror as an optical element to be included, instead of an fθ lens, in an optical scanning device for use in a laser printer or a facsimile machine. A laser beam from a deflector reflects on the spherical mirror and then concentrates on a photoreceptor surface. Through the application of the spherical mirror, the scanning speed of the laser beam is corrected such that the scanning speed becomes even in the extending direction of a scanning line, and such that favorable distortion characteristics and favorable image surface flatness are obtained over a wide angle of view on the light-concentrating surface.
Meanwhile, the unevenness of the machining by a laser machining device can be suppressed effectively if the laser machining device is configured such that the laser beam can be focused on any position on a scanning line, and such that the laser beam is incident on work as perpendicularly as possible. As mentioned above, in reality, it is difficult to apply an fθ lens to laser machining devices. In view of this, Patent Literature 2 discloses a plurality of mirrors arranged in a manner to form a substantially paraboloidal surface. The plurality of mirrors which serve as optical elements are included, instead of an fθ lens, in an optical scanning device for use in a laser machining device. A laser beam from a deflector reflects on the mirrors and then falls on work. Through the application of the mirrors thus arranged, the laser beam is incident on the work as perpendicularly as possible regardless of a rotation angle of the deflector. In addition, regardless of the rotation angle of the deflector, the length of an optical path from a deflection center to the work can be made as constant as possible, and the laser beam can be continuously focused on the work during the scanning.